An example of a solenoid valve, in which a spool valve and a linear solenoid are assembled together, will be explained with reference to FIGS. 2 and 3.
In the drawings, a normally-closed (N/C) type spool valve is also called as a first three-way valve A, while a normally-opened (N/O) type spool valve is also called as a second three-way valve B.
FIG. 2 shows a normally-closed type solenoid valve, which is composed of the first three-way valve A and the linear solenoid 6 for driving the first three-way valve A. This kind of solenoid valve is known in the art, for example, as disclosed in Japanese Patent No. 4,569,371.
According to the first three-way valve A, in a condition that a spool 3 is moved to a right-hand position by a biasing force of a return spring 4 when power supply to the linear solenoid 6 is cut off;                a communication between a first input port A1 for receiving oil pressure from an oil-pressure generating source (not shown) and a first output port A2 is shut off; and        a discharge port 10 is communicated to the first output port A2.        
The first three-way valve A has a first F/B chamber A7 for pushing back the spool 3 in a right-hand direction, when output oil pressure (that is, oil pressure generated at the first output port A2) is increased.
When the output oil pressure is generated at the first output port A2, such output oil pressure is applied to a large-diameter portion (a first F/B land A5) of a left-hand side of the first F/B chamber A7 and to a large-diameter portion (a first input-port controlling land A4) of a right-hand side of the first F/B chamber A7. Then, a first F/B force is generated in the spool 3 in the right-hand direction depending on a difference of diameters (a difference of areas) of the respective large-diameter portions A5 and A4.
The output oil pressure is decided based on a balance among the following forces;
(i) a driving force of the linear solenoid 6 for driving the spool 3 in the left-hand direction;
(ii) a spring force of the return spring 4 for biasing the spool 3 in the right-hand direction; and
(iii) the first F/B force for pushing the spool 3 in the right-hand direction.
FIG. 3 shows a normally-opened type solenoid valve, which is composed of the second three-way valve B and the linear solenoid 6 for driving the second three-way valve B. This kind of solenoid valve is known in the art, for example, as disclosed in Japanese Patent Publication No. 2009-115289.
According to the second three-way valve B, in the condition that the spool 3 is moved to the right-hand position by the biasing force of the return spring 4 when power supply to the linear solenoid 6 is cut off;                a second input port B1 for receiving oil pressure from the oil-pressure generating source (not shown) and a second output port B2 are communicated to each other; and        a communication between the second output port B2 and a discharge port 10 is shut off.        
The second three-way valve B has a second F/B chamber B7 for pushing back the spool 3 in a left-hand direction, when output oil pressure (that is, oil pressure generated at the second output port B2) is increased.
When the output oil pressure is generated at the second output port B2, such output oil pressure is applied to a large-diameter portion (a second F/B land B5) of a right-hand side of the second F/B chamber B7 and to a large-diameter portion (a second input-port controlling land B4) of a left-hand side of the second F/B chamber B7. Then, a second F/B force is generated in the spool 3 in the left-hand direction depending on a difference of diameters (a difference of areas) of the respective large-diameter portions B5 and 34.
The output oil pressure is likewise decided based on a balance among the following forces;
(i) a driving force of the linear solenoid 6 for driving the spool 3 in the left-hand direction;
(ii) a spring force of the return spring 4 for biasing the spool 3 in the right-hand direction; and
(iii) the second F/B force for pushing the spool 3 in the left-hand direction.
According to the prior arts, however, since the solenoid valve of the N/C type and the solenoid valve of the N/O type are separately formed from each other, those solenoid valves lack versatility.